Transconductance amplifiers are frequently used in applications where a controlled current must be delivered to a load, especially a capacitive load. Transconductance amplifiers have substantially low voltage gain characteristics that are beneficial in aiding output voltage stability in negative feedback loop situations. A problem in utilizing transconductance amplifiers is that external performance requirements often necessitate design tradeoffs in the amplifier between important parameters such as high output swing, low output impedance, and low quiescent current.
Amplifiers need a large output swing when driving switches because in order to turn the switches on or off the output of a transconductance amplifier must be relatively high (very near Vcc) while, on the other end (such as under a drop out condition, which is well known by those skilled in the art) the output of a transconductance amplifier needs to go as low as possible.
Transconductance amplifiers often drive a capacitive load. The slew rate of the amplifier output is determined by: t.sub.slew =r.sub.o *C, where r.sub.o equals the output impedance of a transconductance amplifier and C equals the capacitance of the capacitive load. It is often desirable for the slew rate to be as low as possible so that the load capacitance will change states as quickly as possible. Therefore, to minimize the slew rate one needs to provide a low output impedance for the transconductance amplifier. However, maximizing the design of a transconductance amplifier to suit low output impedance considerations is in conflict with the desire for a transconductance amplifier to have high output swing. Typical amplifier designs use an open drain output configuration to maximize the output swing, yet an open drain output configuration has a large output impedance. A standard source follower type output configuration, although providing low output impedance suffers from low output swing. Further, maximizing the design of a transconductance amplifier for minimum slew rate is also in conflict with the desire for a transconductance amplifier to have as low a quiescent current as possible.
Low quiescent current is desirable in order to minimize the power efficiency loss of the system circuitry. However, the output impedance of a transconductance amplifier is inversely proportional to its current. Minimizing design choices for low output impedance necessitates increasing quiescent current. These conflicts in transconductance amplifier performance parameters has limited the use of transconductance amplifiers in applications that have required high output swing, low output impedance, and low quiescent current.
What is needed and what is desirable, therefore, is a transconductance amplifier free from the past constraining design tradeoffs. It is accordingly an object of the invention to provide an improved transconductance amplifier.
It is a further object of the invention to provide a transconductance amplifier having low quiescent current, high output swing, and low output impedance. Other objects and advantages of the invention will become apparent to those of ordinary skill in the art having reference to the following specification together with the drawings herein.